Method of making selenium rectifier



United States Patent METHOD OF MAKING SELENIUM RECTIFIER Jan Coenraad van Vessem, Eindhoven, Netherlands, as-

signor, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware No Drawing. Application May 1'1, 1955 Serial No. 507,748

Claims priority, application Netherlands May 12, 1954 3 Claims. (Cl. 2925.3)

This invention relates to methods of making selenium rectifiers comprising a cadmium-containing counterelectrode.

In a known method, a supporting plate is successively coated with a selenium layer and a counter-electrode consisting of a cadmium-containing alloy, after which the assembly is subjected to a thermal and an electric forming treatment. The thermal and the electric treat ments are usually carried out simultaneously; frequently the heat treatment is directly due to the temperature increase caused by the electric forming treatment. In this method, breakdown, so-called flashing, often occurs due to local defects or overheating. It is also known to immerse the rectifier plates in oil during the forming treatment with the resultant advantage that the possibility of overheating and resultant melting of the counter-electrode is avoided. Moreover, With this method, flashing occurs less frequently. However, this method is laborious, since on termination of the process the oil which might interfere With the making of satisfactory electric contact must be removed.

The chief object of the invention to provide a new method which avoids these disadvantages and which enables the electric forming treatment to be carried out without risk of overheating.

The invention is based on the realization that the electrical properties of the rectifier plates are materially governed by the formation of a chemical compound of the selenium and the cadmium of the counter-electrode, and that the rectifier must not be subjected to excessive electric current before this compound has been allowed to form. According to the invention, the rectifier plate, after the application of the counter-electrode, is heated in a protective environment without an appreciable electric voltage being applied to it to produce the desired selenium-cadmium compound, after which it is formed electrically. The term protective environment as used herein is to be understood to mean an environment, preferably a gas atmosphere, which does not react with cadmium, for example, a rare gas such as argon or helium, or nitrogen, hydrogen, and other reducing gases or mixtures. As an alternative, a vacuum can be used.

' The term appreciable voltage as used herein means that voltage or below which, when applied to the rectifier, produces such low losses in the plate that these losses alone do not cause the temperature of the plate to increase by more than 20 C.

The heat treatment is preferably carried out at a temperature between 75 C. and the melting point of the alloy constituting the counter-electrode. Frequently, cadmium alloys which melt at 103, 144 and 178 C. are

ice

employed; The lower'temperature limit of 75 C. is not too critical, but obviously the temperature should be such that the reaction velocity is sufficiently rapid. The time of treatment may be varied between 10 minutes and 5 hours.

The invention will now be described more fully with reference to a particular example.

On a supporting plate which consists of nickel-plated aluminum or iron, a suspension of carbon in water is sprayed. The carbon is fired to the supporting plate by heating to a comparatively high temperature, for example, 500 C. On this plate a thin layer of amorphous selenium is applied by any known method, such as, for example, flowing, spraying or deposition from a vapor. By means of a known annealing heat treatment, e.g., 140 C., which may be combined with pressure, e.g., 10 kg./cm. under a press, the selenium is converted to the crystalline state. Then, the counter-electrode is applied by spraying. The following alloys have proved very suitable (composition in percent by Weight).

III

Thereafter, the plates are heated in a furnace for half an hour to a temperature of approximately 10 C. below the melting point of the counter-electrode alloy at atmospheric pressure in an environment consisting of a mixture of 30% of hydrogen and of nitrogen, so-called mixed gas. This produces the desired seleniumcadmium compound.

Then, the plates are subjected to the electric forming treatment which consists in the application to the plates of a gradually increasing alternating voltage through a series resistance. As is usual, the maximum value is chosen in accordance with the desired final result. Typical values are, for example, A.-C. voltages of 55 R.M.S. volts applied for 3 hours.

It is found that during the electric formation, local breakdown, the so-called flashing, is substantially obviated by following the method of the invention.

While I have described my invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A method of making selenium rectifier plates comprising an annealed selenium layer and a counter-electrode constituted of a cadmium-containing alloy, which comprises heating the rectifier plate containing the annealed selenium layer and the counter-electrode in a protective environment without an appreciable electric voltage being applied thereto, and thereafter subjecting the plate to an electric forming treatment.

2. A method of making selenium rectifier plates corngas atmosphere Without an appreciable electric voltage being applied thereto to form a selenium-cadmium compound, and thereafter subjecting the plate to an electric forming treatment.

3. A method of making selenium rectifier plates comprising an appealed selenium layer and a counter-electrode constituted of a cadmium-containing alloy, which comprises heating the rectifier plate containing the annealed selenium layer and the counter-electrode in a protective environment Without an appreciable electric voltag'efbeing' applied thereto to form a selenium-cadmium compound, and thereafter subjecting the plate to an electric forming treatment by applying thereto an alternating voltage of gradually-increasing magnitude.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF MAKING SELENIUM RECTIFIER PLATES COMPRISING AN ANNEALED SELENIUM LAYER AND A COUNTER-ELECTRODE CONSTITUTED OF A CADMIUM-CONTAINING ALLOY, WHICH COMPRISES HEATING THE RECTIFIER PLATE CONTAINING THE ANNEALED SELENIUM LAYER AND THE COUNTER-ELECTRODE IN A PROTECTIVE ENVIRONMENT WITHOUT AN APPRECIABLE ELECTRIC VOLTAGE BEING APPLIED THERETO, AND THEREAFTER SUBJECTING THE PLATE TO AN ELECTRIC FORMING TREATMENT. 